1. Field of the Invention
The present invention relates to a scanning optical device and image forming apparatus and, more particularly, to a compact image forming apparatus suitable for high-resolution printing with, e.g., a color electrophotographic process, such as a laser beam printer (LBP) or color digital copying machine, in which an imaging position variation caused by the wavelength fluctuation of a light source means, an aberration variation upon an environmental fluctuation, and the like in each scanning optical device are compensated by a compensation means in a color image forming apparatus having a plurality of scanning optical devices, thereby suppressing an imaging position deviation in especially the main scanning direction, and reducing color misregistration, and image density nonuniformity at low cost with a simple arrangement.
2. Related Background Art
Conventionally in a scanning optical device used in an image forming apparatus such as a laser beam printer or digital copying machine, a beam modulated and emitted by a light source means in accordance with an image signal is periodically deflected by an optical deflector formed from, e.g., a rotary polyhedral mirror (polygon mirror), and converged into a spot on a photosensitive recording medium (photosensitive drum) surface by an imaging optical system (scanning optical system) having an f-xcex8 characteristic. This surface is optically scanned to record an image.
FIG. 24 is a schematic view showing the main part of a conventional scanning optical device.
In FIG. 24, a divergent beam emitted by a light source means 91 is converted into a nearly collimated beam by a collimator lens 92. The beam is limited by a stop 93, and enters a cylindrical lens 94 having a predetermined refracting power in only the sub scanning direction. Of the nearly collimated beam that enters the cylindrical lens 94, light components in the main scanning plane directly emerge as a nearly collimated beam. In the sub scanning plane, light components are converged to form an almost line image on a deflection surface (reflection surface) 95a of an optical deflector 95 formed from a rotary polyhedral mirror (polygon mirror).
The beam deflected and reflected by the deflection surface 95a of the optical deflector 95 is guided onto a photosensitive drum surface 98 as a surface to be scanned via an imaging optical element (f-xcex8 lens) 96 having f-xcex8 characteristics. By rotating the optical deflector 95 in a direction indicated by an arrow A, the beam scans the photosensitive drum surface 98 in a direction indicated by an arrow B to record image information.
FIG. 25 is a schematic view showing the main part of a conventional color image forming apparatus. In FIG. 25, the color image forming apparatus simultaneously uses a plurality of scanning optical devices 111 to 114 like the above-described one to record pieces of image information of respective colors on corresponding photosensitive drums 121 to 124, thereby forming a color image. In this color image forming apparatus, it is important to reduce not only an imaging position deviation represented by jitter of a single color but also scanning line deviations (to be referred to as xe2x80x9cmisregistrationxe2x80x9d hereinafter) between colors, and image density nonuniformity between colors in order to align a plurality of scanning lines and form an image. For this purpose, the scanning optical device must satisfy the following conditions.
(1) An imaging position variation (spot position deviation) in the main scanning direction along with the wavelength fluctuation of a beam emitted by a semiconductor laser serving as a light source (to be simply referred to as the xe2x80x9cwavelength fluctuation of the semiconductor laserxe2x80x9d hereinafter) is compensated.
(2) A focus variation in the sub scanning direction along with an environmental fluctuation such as influential temperature elevation is compensated (a focus variation in the main scanning direction is originally small and does not pose any problem in many cases).
(3) An imaging position deviation in the main scanning direction along with an environmental fluctuation such as temperature elevation is small.
(4) An imaging position deviation in the main scanning direction caused by a wavelength variation (not accompanied by any environmental fluctuation) is small.
The image forming apparatus requires not only stable optical performance in one scanning optical device (single color) but also an arrangement which can prevent misregistration or image nonuniformity between colors even when the light source wavelength (wavelength of a beam emitted by the light source), initial wavelength, or use environment (particularly ambient temperature) of the scanning optical device fluctuates, or even when the light source wavelengths, initial wavelengths, or use environments of a plurality of scanning optical devices differ.
Further, when such a color image forming apparatus uses a light source having a plurality of light-emitting portions represented by multi-beam lasers (e.g., multi-semiconductor lasers), jitter by a plurality of beams (scanning interval fluctuation in the main scanning direction on the photosensitive drum surface) occurs owing to the wavelength difference between the light-emitting portions, greatly degrading the image quality. To prevent this, (1) compensation of a spot position deviation and (4) compensation of a spot imaging position deviation not accompanied by any environmental fluctuation become more important.
Such a scanning optical device has conventionally used a glass lens or glass mirror which hardly exhibits a characteristic fluctuation caused by an environmental fluctuation. However, an aberration variation caused by the wavelength fluctuation of a semiconductor laser remains, advanced aberration correction by an aspherical surface cannot be performed, and the cost is high. Thus, demands have arisen for compensation of a wavelength fluctuation, environmental fluctuation, or the like by a scanning optical device using a plastic material.
Japanese Patent Application Laid-Open No. 3-231218 discloses a scanning optical device in which a scanning optical system is constituted by a glass spherical lens and plastic toric lens. In this reference, an imaging position deviation in the main scanning direction caused by a wavelength variation of 5 nm is 64.6 xcexcm, and a focus deviation in the sub scanning direction at a temperature elevated by 25xc2x0 C. is +1.7 mm. For example, a color image forming apparatus using a plurality of such scanning optical devices suffers misregistration or image nonuniformity between colors.
Japanese Patent Application Laid-Open No. 7-128603 discloses an example in which a glass lens and glass cylinder mirror are used as the scanning optical system of a scanning optical device used in a color image forming apparatus. In this reference, all the scanning optical systems are made of a glass material. An aberration variation caused by the wavelength fluctuation of a semiconductor laser remains. Aberration correction by an aspherical surface cannot be performed, so the optical path is long. In addition, the cost is high.
Japanese Patent Application Laid-Open No. 10-232347 discloses an example in which a scanning optical system is formed from two toric lenses. In this reference, all the scanning optical systems are made of a plastic, which is advantageous for aberration correction. However, the laser wavelength increases as the temperature is elevated. When the refractive index of the scanning optical system decreases, the laser wavelength varies in a direction in which the focal length of a scanning lens increases. As a result, the imaging position in the main scanning direction greatly deviates.
It is an object of the present invention to provide a compact scanning optical device and color image forming apparatus suitable for high-resolution printing in which an imaging position variation caused by the wavelength fluctuation of a light source means, an aberration variation upon an environmental fluctuation, and the like in each scanning optical device are compensated by a compensation means (third optical element) in a color image forming apparatus having a plurality of scanning optical devices, thereby suppressing an imaging position deviation in especially the main scanning direction, and reducing color misregistration and image density nonuniformity at low cost with a simple arrangement.
It is another object of the present invention to provide a scanning optical device and color image forming apparatus suitable for high-resolution printing in which an aberration variation accompanied by the environmental fluctuation of each scanning optical device and an aberration variation not accompanied by any environmental fluctuation are suppressed in a color image forming apparatus having a plurality of scanning optical devices, thereby reducing color misregistration and image density nonuniformity at low cost with a simple arrangement.
According to one aspect of the invention, a scanning optical device comprises light source means, and an optical element having a refraction element and a diffraction element,
wherein powers of said refraction element and said diffraction element are set such that an imaging position deviation in a main scanning direction on a surface to be scanned that is caused by a wavelength variation of a beam emitted by said light source means along with an environmental fluctuation of said scanning optical device, and an imaging position deviation in the main scanning direction on the surface to be scanned that is caused by a refractive index variation of a material of said optical element along with an environmental fluctuation are set to be in opposite directions at two ends of an effective scanning region.
According to further aspect of the invention, said optical element includes an element which forms the beam emitted by said light source means into an image on the surface to be scanned, and letting xcfx86Lm and xcfx86Dm be powers of said refraction element and diffraction element constituting said optical element in the main scanning direction, and xcexdL be an Abbe number of a material of said refraction element, the powers satisfy a condition:
xcexdL/6.90xe2x89xa6xcfx86Lm/xcfx86Dmxe2x89xa6xcexdL/3.45
According to further aspect of the invention, an imaging position variation per unit mode hopping of said light source means is not more than 3 xcexcm at the two ends of the effective scanning region.
According to further aspect of the invention, a focus position deviation in a sub scanning direction on the surface to be scanned that is caused by the wavelength variation of the beam emitted by said light source means along with the environmental fluctuation, and a focus position deviation in the sub scanning direction on the surface to be scanned that is caused by the refractive index variation of the material of said optical element along with the environmental fluctuation are substantially compensated within the effective scanning region.
According to further aspect of the invention, the environmental fluctuation includes an atmospheric variation of said scanning optical device and a temperature variation caused by self-temperature elevation.
According to further aspect of the invention, said refraction element includes a toric lens, and said diffraction element includes a composite optical element obtained by integrating a refraction surface and a diffraction surface.
According to further aspect of the invention, said refraction element and diffraction element constituting said optical element are formed from a plastic material.
According to further aspect of the invention, said light source means includes a multi-beam laser source having a plurality of light-emitting portions.
According to further aspect of the invention, letting xcfx86Ls and xcfx86Ds be powers of said refraction element and diffraction element constituting said optical element in a sub scanning direction, the powers satisfy a condition:
1.0xe2x89xa6xcfx86Ls/xcfx86Dsxe2x89xa62.6
According to further aspect of the invention, the powers of said refraction element and said diffraction element are set to compensate for an imaging position variation in the main scanning direction on the surface to be scanned that is caused by a wavelength fluctuation of the beam emitted by said light source means.
According to one aspect of the invention, an image forming apparatus comprises a plurality of pairs of scanning optical devices defined in claim 1 and corresponding image carriers, guides beams emitted by said scanning optical devices to corresponding image carriers, scans surfaces of said image carriers to form images of different colors on the surfaces of said image carriers, and forms a color image by the images formed on the surfaces of said image carriers.
According to one aspect of the invention, a scanning optical device guides a beam emitted by light source means to a surface to be scanned, and scans the surface with the beam,
wherein said scanning optical device comprises compensation means having a refraction element and a diffraction element so as to compensate for an imaging position deviation in a main scanning direction on the surface to be scanned that is caused by a wavelength fluctuation of the beam emitted by said light source means, and
letting xcfx86Lm and xcfx86Dm be powers of said refraction element and said diffraction element in the main scanning direction, the powers satisfy a condition:
5.8xe2x89xa6xcfx86Lm/xcfx86Dmxe2x89xa620
According to further aspect of the invention, a focus position variation in a sub scanning direction on the surface to be scanned that is caused by an environmental fluctuation of said scanning optical device is compensated by power variations of said refraction element and said diffraction element, and a wavelength variation of the beam emitted by said light source means.
According to further aspect of the invention, said refraction element is formed from a plastic material.
According to further aspect of the invention, said compensation means includes means for forming the beam emitted by said light source means into an image on the surface to be scanned.
According to further aspect of the invention, said light source means includes a multi-beam laser source having a plurality of light-emitting portions.
According to further aspect of the invention, said compensation means includes means for compensating for an imaging position variation in the main scanning direction on the surface to be scanned that is caused by the wavelength fluctuation of the beam emitted by said light source means, and a scanning interval fluctuation in the main scanning direction on the surface to be scanned that is caused by a wavelength difference between beams emitted by a plurality of light-emitting portions.
According to one aspect of the invention, an image forming apparatus comprises a plurality of pairs of scanning optical devices set out in the foregoing and corresponding image carriers, guides beams emitted by said scanning optical devices to corresponding image carriers, scans surfaces of said image carriers to form images of different colors on the surfaces of said image carriers, and forms a color image by the images formed on the surfaces of said image carriers.
According to one aspect of the invention, an image forming apparatus comprises a scanning optical device set out in the foregoing and a printer controller for converting code data input from an external device into an image signal and inputting the converted image signal to said scanning optical device.